Build/Test Tools: Measure additional load time metrics in performance tests.

Three new metrics are being collected and reported as part of this change:

- Time To First Byte (TTFB) - the time between the request for a resource and when the first byte of a response begins to arrive
- Largest Contentful Paint (LCP) — the render time of the largest image or text block visible within the viewport
- The difference between the two (LCP minus TTFB)

Props joemcgill, flixos90, oandregal, mukesh27, youknowriad, swissspidy.
Fixes #58360.
Built from https://develop.svn.wordpress.org/trunk@56399


git-svn-id: http://core.svn.wordpress.org/trunk@55911 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.4-alpha-56398';
+$wp_version = '6.4-alpha-56399';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.